The present invention relates to apparatus for reading information stored in a memory layer, such as an X-ray memory layer, and an X-Ray cassette for use with the apparatus.
Particularly for medical purposes, an image of an object (such as a patient) is created using X-ray radiation that is stored in a memory layer as a latent image. A phosphorus film base is often used for this memory layer. In order to read the X-ray image stored in the memory layer, the memory layer is excited using a radiation source. Because of this excitation, the memory layer radiates light of an intensity corresponding to the stored X-ray image. The light emitted from the memory layer is received by a receptor and eventually converted into electrical signals so that the X-ray image stored in the memory layer can then be made visible. The X-ray image can, for example, be projected directly on a monitor or printed onto a special photographic X-ray film suitable for X-ray images.
Such a device used to read out information stored in a memory layer is known, for example, from published International Patent Application No. WO 99/28765. In this known device, the memory layer is excited by rows by an exciter beam that is created by a radiation source. This radiation source may be a laser diode strip, for example. The light emitted because of excitation of the memory layer is received by a receptor. For this purpose, the receptor contains a number of light-sensitive surfaces that are arranged in adjacent rows. The radiation emitted from the memory layer is received by the light-sensitive surfaces. The receptor may be a Charge-Coupled-Device (CCD) that contains a number of photo-detectors arranged in adjacent rows. These light-sensitive surfaces are usually symmetrical.
It is known from the aforementioned publication No. WO 99/28765 that the size of the excited points on the memory layer is dependent upon the size of the light-sensitive surfaces of the individual photo-detectors of the receptor. To ensure a high degree of focus and high resolution of the X-ray image read from the memory layer, the size of the individual points should be kept small. This is particularly necessary for mammography applications. In the known apparatus, a projection device can be placed between the memory layer and the receptor by means of which the emission radiation emitted from excited rows of the memory layer is projected onto the receptor. So-called Selfoc lenses or micro lenses may be used as a projection device. These have small dimensions, so that the separation between the memory layer and the receptor can be kept small. The optical input surfaces and the Selfoc or micro lenses are symmetrical.
Based on the principle advanced by the aforementioned publication No. WO 99/28765, the principal objective of the present invention is to enable improved quality during the reproduction of information stored in a memory layer.
This objective, as well as other objectives which will become apparent from the discussion that follows, are achieved, in accordance with the present invention, by providing the receptor with light sensitive services arranged in adjacent rows, wherein the dimension of the light sensitive surfaces arranged perpendicular to the orientation of the rows is greater than that oriented along the row direction. Alternatively or additionally, the apparatus can include an optical projection device which projects the emission radiation onto the receptor. The numerical aperture of the projection device with respect to the excited row arranged perpendicular to the orientation of the rows is made larger than the aperture along the row direction.
This invention achieves a high degree of focus during projection of the emission radiation emitted from the memory layer to the receptor, and high collection efficiency during collection of the radiation emitted by the memory layer. Based on this invention, the light-sensitive surfaces of the receptor or the aperture angle at which the optical projection device receives the radiation emitted from the memory layer are asymmetrical. The invention is based on the surprising fact that a high degree of focus during projection of the information onto the receptor in the direction of the excited rows may be ensured, and simultaneously that this good focus need not necessarily be maintained during projection of the information onto the receptor perpendicular to the orientation of the row. According to the arrangement based on the invention, the quantity (and thereby the intensity) of the emission radiation received by the receptor perpendicular to the orientation of the row is increased in comparison with a symmetrical configuration. In this manner, an improved signal-to-noise ratio is achieved during reception of the information.
In a particularly advantageous embodiment of the invention, both the light-sensitive surfaces of the receptor and the aperture angle of the optical projection device (with respect to the excited rows) are asymmetrical. Thus, the information stored in the memory layer may be more accurately projected onto the receptor. Reproduction of this information may therefore be qualitatively further improved.
In a further particularly advantageous embodiment of the invention, the light-sensitive surfaces of the receptor are so arranged that they are two to eight times (in particular, two to four times) as large perpendicular to the orientation of the rows as they are parallel to the orientation of the rows. These dimensions allow an optimum signal-to-noise ratio while maintaining a high degree of focus of information projection onto the receptor.
The optical projection device can be advantageously arranged that, during the projection of the emission radiation onto the receptor, the orientation of the information read during projection onto the receptor along the row is the same as the orientation of the stored information in the memory layer. Thus, using matched optical components, projection of a non-inverted image of the information read onto the receptor is ensured. It is also possible to create the non-inverted image by the use of suitable electronic image data processing. This, however, requires a large amount of computing power that can be avoided by the use of the optical projection device.
Further, the optical projection device can advantageously also include a filter to prevent excitation radiation used to excite the memory layer from being received by the receptor. The reproduction quality of the stored information is thereby further improved. Distortion during reproduction of the information caused by the detection of excitation radiation by the receptor would be prevented.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.